At the present time, most power distribution is achieved using high voltage alternating current (AC) such that resistive losses in long power distribution lines can be reduced to acceptable levels. Alternating current allows high voltage power at reduced currents to be converted to a lower voltage and higher current near the location where the power will be consumed by the simple expedient of a transformer. However, most electronic devices or other apparatus that includes such electronic devices as well as power storage devices such as batteries require direct current (DC) power at a closely regulated voltage. DC power can be easily obtained from AC power by rectification using diodes or the like and filtering to obtain an approximate DC voltage from which the required DC voltage may be obtained. Maintaining a DC voltage at a particular desired level within a small tolerance, however, generally requires regulation by an active circuit since practical filters generally cannot hold a DC voltage within tolerances generally required by electronic devices.
While analog voltage regulators have been known for many years, they are inefficient to provide any significant current because of the voltage drop developed across them and the power they consume for that reason. Accordingly, at the present time, switching regulators and power converters such as inverters are much preferred because of the increased efficiency they can potentially provide by rapidly connecting and disconnecting the power input thereto as required to maintain the output voltage at a desired level. Thus, the period when a voltage drop occurs across the regulator or power converter will be limited to relatively short periods of time and overall efficiency can be improved.
However, such switching is necessarily cyclic and some portions of the switching cycle may be less efficient than others due to the instantaneous conduction conditions in the regulator or power converter circuit. The amount of current passing through the regulator or power converter can vary widely over a single switching cycle and between switching cycles, depending on input voltage or output current.
For example, at light loads where conduction losses are reduced, the amount of power consumed by the rapid switching, referred to as switching losses, becomes a significant fraction of the total power consumption of the regulator or power converter. Switching losses can be substantially eliminated by so-called soft switching using MOSFETs such that the internal diode (referred to as a body diode) of the MOSFET will begin to conduct before the MOSFET is switched on. Conversely, at heavy loads where high current is drawn through the regulator or power converter, conduction losses in the switches, generally MOSFETs since they are well-suited to operation in a soft-switching mode, tend to predominate because the conduction path in MOSFETs is substantially resistive. Conduction losses theoretically could be mitigated by placing a large number of MOSFETs in parallel to reduce currents in individual MOSFETs and thus reduce the voltage drop. However such a parallel connection of many switches is not generally practical due to the cost of multiple switching devices.
Other attempts to improve efficiency of switching devices in regulators and power converters by control of mode of operation have yielded only marginal improvements. Such attempts to tailor mode of operation to load levels are necessarily complex, increasing cost and reducing power density and require some amount of power for operation as well as load sensing. Further, such arrangements have not been able to alter modes of operation within a single switching cycle.
Some of the above causes of inefficiency are particularly intractable in regard to high power inverters which produce AC current from DC input power. While inverters have been known, particularly for low-power applications, they have become of increasing interest in recent years in connection with renewable or so-called “green” energy sources such as solar power or wind-powered generators where conditions for generation of power is necessarily intermittent and energy must be stored in some form such as charge in batteries. Energy stored in such a manner will necessarily be available as a DC voltage which may require conversion to AC power for distribution.
Thus, the inverter output voltage and current will ideally vary sinusoidally at the desired line frequency and the current through the switches on each side of the inverter circuit will vary from zero to a maximum and back to zero during respective half-cycles of the line frequency with the maximum current depending ultimately on the load. Thus, it can be clearly seen that the full range of output current that may be required by the load will be carried by the respective switches of the inverter during respective half-cycle periods of the output and improvements in efficiency have been limited since both light and heavy loads must be accommodated in a single half-line cycle.